The present invention relates generally to fluid absorption, adsorption and separation devices, and more particularly to hollow fibers having an axial slit-shaped capillary along a substantial section of the fiber's length that provides an entrance from the fiber exterior to at least one internal fiber compartment. Such fibers act as very high efficiency absorptive materials, as well as high-surface-area fluid separation devices.
Absorbent fibers in the form of hollow fibers and solid fibers with various cross-sectional shapes have found use in numerous health and industrial applications, such as, towels, diapers, feminine napkins, wound dressings and spill clean-up. Fibers with high absorption capacity have been described, for example, in U.S. Pat. Nos. 4,707,409; 5,124,205; 5,200,248; 5,268,229; 5,496,627; 5,972,505; 5,977,429; 6,093,491; and 6,296,8211.
In addition, there is patent literature utilizing various fiber cross-sections encompassing features, such as, ribs, wings, lobes, grooves and channels on the exterior of the fiber to adsorb liquids. For example Phillips et al. describe fibers with spines and arms (U.S. Pat. No. 6,890,436) to adsorb and transport aqueous liquids. Other patents describe “C”-shaped (U.S. Pat. No. 5,156,831) and multi-lobal cross-sections (U.S. Pat. No. 3,220,173, U.S. Pat. No. 4,648,830, U.S. Pat. No. 5,057,368, U.S. Pat. No. 5,154,908, and U.S. Pat. No. 6,379,564), which can be filled with a liquid or particulates (U.S. Pat. No. 6,379,564) that are able to filter or treat a liquid that surrounds the fiber. However, it should be noted that in order to hold the liquid or particulates in the fiber cavities, the lobes of the multi-lobed fibers must be sized small enough to hold these materials in the fiber by capillary forces. Thus, the capillary in these fibers is the internal cavity of the fiber.
Since the cavity holding the liquid or particulates is formed by lobes or circumferential extensions on the lobes, this limits the maximum potential diameter of the fiber, which in turn limits the internal capacity of the fiber. In addition, since it is very difficult to extrude a multi-lobe fiber with narrow separation between the lobes due to polymer rheology constraints, they are usually limited to three or a maximum of four lobes.
In the prior art there are also hundreds of patents in the literature that describe the use of permeable hollow fibers to separate or purify fluids based on permeability through the walls of hollow fibers. These fibers are used principally in the dialysis of blood (U.S. Pat. No. 7,524,417), gas separation (U.S. Pat. No. 6,755,894), the purification of water (U.S. Pat. No. 6,890,436) and blood (U.S. Pat. No. 7,608,189), as well as in the separation of blood components (U.S. Pat. No. 5,846,427).
Since these fibers function on the basis of a permeable wall, there are many applications where they cannot be used. For example, in separating immiscible liquids from one another. Thus, a need exists to be able to separate one fluid from another on the basis of surface tension and wettability. This will require a non-porous hollow fiber with a specific surface composition, with high internal capacity, and numerous capillary entrances so that access to the internal cavities will be rapid. This is not possible with the prior art.
The present invention describes fibers with an axial capillary slit that behaves physically as if a capillary existed along the entire length of the fiber. Such a capillary slit along the entire length of a hollow fiber, instead of a capillary opening in only the ends of a hollow microscopic fiber, improves the efficiency and rate of fluid entering the hollow fiber by many orders of magnitude. This greatly increases the usefulness of these fibers over the prior art in the areas of adsorption, absorption and fluid separation.
U.S. patent application Ser. No. 10/435,008, titled “Separation Devices, (abandoned), describes separation devices which can separate fluids according to how they wet the inner walls of capillaries, as well as their chemical, electrical or magnetic selectivity. For a fluid that does not wet a particular capillary wall, the minimum cross-sectional dimension of that capillary can also be used as a separation mechanism because the pressure needed to force a non-wetting liquid into the capillary depends on its minimum cross-sectional dimension. That is, the pressure (Pc) required to force a non-wetting fluid into a cylindrical capillary is dependent on the minimum cross-sectional radius (rc), the surface tension of the liquid (γ) and the contact angle (θ) that the liquid makes with the material that it is exposed to on the inner wall of the capillary. This dependence is expressed by the equation:Pc=2γ cos θ/rc  (1)
For highly non-circular capillaries such as slits, this equation can be generalized to:Pc=2γ cos θ/d  (2)where the radius has now been replaced by (d) which is the minimum slit dimension.
Fluid separation devices based on admittance/exclusion are described in the Separation Devices patent application. In those devices, a fluid stream or mixture that impinges on the ends of the capillaries at the entrance face of the fluid separation device can be separated on the basis of the exclusion of one or more components of the fluid stream or mixture by certain capillaries in the fluid separation device entrance face. This selective exclusion from discrete capillaries in the separation device face can be used to separate the components of two phase flows.
To function as a fluid separation device and separate fluids on the basis of their exclusion from certain capillaries, it is necessary that the different capillaries in the fluid separation device differ from one another in respect to at least one separation characteristic, such as their cross-sectional dimensions, wettability, chemical characteristics, electrical characteristics and magnetic characteristics. Except for dimensional differences, these separation characteristics arise from the character of the inner surface of the capillary slit and inner wall of the capillary, which depends on the material(s) used to form these surfaces, any coating(s) on these surfaces or any modification(s) to the material(s) forming these surfaces, such as might be made by mechanical, chemical, physical, radiation or energetic particle means.
Thus, to function as a fluidic separation device based on admittance/exclusion, at least one of the capillaries in the separation device must possess at least one characteristic necessary to separate at least one of the fluids in the incident fluid stream or mixture from the others. That is, the device must possess at least one capillary that allows the entrance of at least one of the fluids in the stream or mixture and at the same time excluding at least one other component in the fluid stream or mixture. In addition, all the capillaries in the separation device that are able to admit a certain fluid should terminate at a precise position on the exit surface of the separation device, such that the effluent of all these capillaries is in common. This effluent can then be collected or can enter another separation device for further processing.
The example embodiments described in the Separation Devices patent application Ser. No. 10/340,381, issued on Jan. 3, 2006 as U.S. Pat. No. 6,982,787, are a clear advance over the prior art. Yet, further improvements over the prior art are possible and desirable. For, example it is desirable to increase the rate of adsorption of a single species, the rate of separation of at least two immiscible fluids as well as to increase the capacity of these processes.
It is, therefore, an object of the present invention to build on the teachings of the Separation Devices patent application to provide better and more efficient fluid separation, enhanced fluid capacity, increased rate of separation and other functions.
It is a feature of the present invention that it will find valuable use for separating immiscible liquids such as fat, oils and water from one another.
It is another feature of the present invention that it will find valuable use for the removal of gaseous species from liquids, such as removing oxygen from jet fuel to increase turbine engine temperatures and efficiencies.
It is a further feature of the present invention that it will find valuable use for “Dry Feel” fabrics.
It is an advantage of the present invention that its ability to separate immiscible liquids will find valuable use for soaking up spills generally, and particularly for such important uses as cleaning up oil spills at sea.
It is another advantage of the present invention that it will improve hygiene adsorbents such as are used in diapers and tampons.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.